1. Field of the Invention
The present invention relates to a lithium ion conductive solid electrolyte. In particular, the invention relates to a solid electrolyte which is suitable for a wholly solid lithium primary battery or a wholly solid lithium ion secondary battery, a lithium primary battery and a lithium ion secondary battery.
2. Description of the Related Art
In recent years, a lithium ion battery is widely utilized as an electric power source of portable information devices such as cellular phones or laptop personal computers. The battery to be used for the foregoing applications is demanded to have high safety and simultaneously required to have a high energy density and excellent cycle properties.
In order to cope with these demands, as to an electrolyte material, a gel polymer electrolyte having a polymer impregnated with an electrolytic liquid from an organic electrolytic liquid gets attention. A gel polymer battery utilizing a gel polymer electrolyte uses an electrolyte in a gel form prepared by impregnating an electrolytic liquid as a liquid in a polymer and involves advantages such that liquid leakage hardly occurs; that safety of the battery is enhanced; and that a degree of shape freedom is high. However, since this gel polymer electrolyte still uses an organic electrolytic liquid, it cannot be said that the risk is completely excluded, and there is involved a possibility of ignition or the like. Accordingly, in order to achieve higher safety, a battery which does not use an organic electrolytic liquid at all is desirable.
Then, as a material which has safety and which is low in a load to the environment, there have been made a number of attempts to use an inorganic solid electrolyte. A solid electrolyte obtained by sintering a powder is proposed for the reasons that it has a high degree of freedom for forming and is advantageous from the standpoint of costs.
However, the solid electrolyte obtained by sintering a powder contains a number of pores, and when it is used as a separator in a lithium metal-air primary battery composed of a lithium metal electrode and an air electrode, the moisture formed in the air electrode passes through the pores existing in the solid electrolyte and reaches the side of the lithium electrode, thereby possibly causing ignition. For that reason, in the lithium metal-air primary battery, a solid electrolyte which is minute and remarkably low in moisture permeation is required. However, a solid electrolyte which is sufficiently low in moisture permeation has not been obtained yet.
Also, in the solid electrolyte obtained by sintering a powder, the pores hinder the conduction of a lithium ion so that it was difficult to attain high lithium ion conductivity. In this point, there is problem as a solid electrolyte for an application to a lithium ion secondary battery or a lithium primary battery.